MANY ANIMALS perform amazing feats of navigation. A magnetic sense that plays an important role in navigation has been documented in dozens of species, eg homing pigeons, sea turtles, robins, lobsters, honeybees, ants, mole rats, elephant seals and bacteria, writes WILLIAM REVILLE
The precise mechanism by which biological organisms perform magnetic navigation is known only for bacteria but researchers are now homing in on how the animal magnetic sense works in general. The current state of play in this field is summarised by Davide Castelvecchi in Scientific American, January 2012.
First, a word on the geometry of the Earth’s magnetic field. Imagine a compass needle suspended from a string, allowing it to rotate in any direction. The needle points in the direction of the earth’s magnetic field – the north pole of the needle points north. The force exerted on the needle is proportional to the intensity of the magnetic field and the inclination (90 degrees up to 90 degrees down) of the needle is its deviation from the horizontal. In the northern hemisphere, the north pole of the needle points downwards, pointing straight down when over the north magnetic pole. It rotates upwards as latitude decreases, is horizontal at the magnetic equator, and continues to rotate upwards in the southern hemisphere until it is pointing straight up at the south magnetic pole.
There is good evidence that birds use their magnetic sense to navigate. Some varieties of European robins migrate southwards annually. In the 1950s researchers noted that caged robins try to escape towards the south in the autumn even in the absence of visual cues indicating where south is. Later, in the 1960s, Wolfgang Wiltschko tricked the robins into trying to escape in the “wrong” direction by manipulating electromagnetic coils mounted on the bird cages.
There is evidence that birds are sensitive not only to the geographic direction of the magnetic poles but also to the inclination of the Earth’s magnetic field, using this latter information to roughly measure distance from the magnetic poles. The idealised description of the Earth’s magnetic field outlined before took no account of local anomalies in direction and strength caused by magnetic minerals on the Earth’s crust. Some animals, eg sea turtles, appear to have a mental map of these anomalies, which they use to navigate.
Castelvecchi emphasises that birds are not solely reliant on their magnetic sense for navigation. For example, homing pigeons get lost if the nerves to their noses are surgically cut, depriving them of their sense of smell. Also, birds raised in aviaries that only open upwards cannot tell from which direction environmental smells are coming and are unable to navigate. Birds and other animals use a variety of cues in navigating – sun, stars, moon, ground landmarks, smells – as well as sensing the Earth’s magnetic field. At any particular time they pick the strongest navigational cues from this repertoire.
The mechanism within cells whereby organisms detect and process magnetic information is understood for sure only in bacteria. Certain marine bacteria at latitudes where the magnetic field inclination is steep sense and use this information to swim downwards towards the muddy seafloor where they prefer to be. These bacteria contain strings of microscopic particles of magnetite, a strongly magnetic form of iron oxide. These particles align with the Earth’s magnetic field, thereby allowing the bacterium to orient in the right direction.
Researchers have looked for similar magnetite-based structures throughout the animal kingdom. They found them in the beaks of homing pigeons but there is disagreement as to whether they are involved here with magnetic reception. However, in cells lining the nasal passage of rainbow trout, it has been shown that magnetite particles respond to changes in the direction of the Earth’s magnetic field by opening channels in the cell membrane, allowing ions (charged atoms) to pass through to initiate a nerve signal to the brain, thereby indicating the direction the fish should swim.
Other fascinating work is under way studying a protein pigment in the retinas of mammals, birds and flies that is affected by changes in the direction of the Earth’s magnetic field to speed up or slow down a chemical reaction. This could signal the brain making the animal aware of the field’s direction. Cattle can sense the Earth’s magnetic field, routinely aligning their bodies with a preference towards the north-south axis. Humans may also have a magnetic sense – no surprise for Franz Anton Mesmer’s ghost, who suggested the notion of “animal magnetism” in the 18th century.
Researchers hope to soon reveal the general mechanisms of magnetic reception. We can then confidently add this sixth new sense to the traditional five.
William Reville is an Emeritus Professor of Biochemistry and Public Awareness of Science Officer at UCC
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